1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a small-sized zoom lens having a high optical performance and having a relatively simple construction, which is most suitable for a camera having a relatively short back focal distance among lens-shutter cameras, such as video cameras, still cameras and digital cameras.
2. Description of Related Art
In recent years, with the advance of working technology for aspherical lenses, there have been proposed a variety of zoom lenses in which a number of aspherical lenses are used for reducing the number of lens elements, shortening the total length of the lens and improving the optical performance.
However, since the aspherical lens is difficult to form as compared with a spherical lens, it is desirable to limit the number of aspherical lenses to the necessary minimum. For that purpose, it is necessary to effectively dispose aspherical lenses in an optical system.
In the meantime, as one type of the simplest zoom lenses, there are known two-unit zoom lenses each composed of two lens units.
Among the two-unit zoom lenses, heretofore, there have been proposed, as zoom lenses for use with a lens-shutter camera which requires no long back focal distance, a variety of two-unit zoom lenses each of which is composed of, in order from an object side, a first lens unit of positive refractive power and a second lens unit of negative refractive power, and in which a variation of magnification is effected by varying an axial air separation between the two lens units.
For example, in Japanese Laid-Open Patent Applications No. Sho 56-128911 (corresponding to U.S. Pat. No. 4,682,860), No. Sho 57-201213 (corresponding to U.S. Pat. No. 4,682,860), No. Sho 60-170816, No. Sho 60-191216 (corresponding to U.S. Pat. No. 4,659,186), No. Sho 62-56917, etc., there have been proposed small-sized zoom lenses each of which is composed of, in order from an object side, a first lens unit of positive refractive power and a second lens unit of negative refractive power, and in which a variation of magnification is effected by varying an interval between the two lens units.
According to the above publications, the positive-and-negative refractive power arrangement in order from an object side is adopted and the appropriate rule of design of each lens unit is set forth, thereby realizing a zoom lens whose back focal distance is relatively short, whose total length is shortened and which has a high optical performance.
Among the above publications, in Japanese Laid-Open Patent Application No. Sho 56-128911, there has been proposed a small-sized zoom lens in which the first lens unit is composed of four lenses, i.e., positive, negative, positive and positive lenses, and the second lens unit is composed of two lenses, i.e., positive and negative lenses, so that the entire lens system is simplified.
On the other hand, in Japanese Laid-Open Patent Application No. Sho 63-311224, there has been proposed a small-sized zoom lens in which the first lens unit is composed of four lenses, i.e., positive, negative, positive and positive lenses, the second lens unit is composed of two lenses, i.e., positive and negative lenses, and an aspheric surface is used in each of the two lens units, thereby improving an optical performance.
In addition, in Japanese Laid-Open Patent Application No. Hei 4-161914, there has been proposed a zoom lens in which the first lens unit is composed of four lenses, i.e., a positive lens, a negative lens, an aspherical lens and a positive lens, the second lens unit is composed of three lenses, i.e., positive, negative and negative lenses, and an aspheric surface is used in each of the three lens units.
Further, in Japanese Laid-Open Patent Applications No. Sho 62-90611, No. Sho 62-113120 and No.
Hei 3-116110, there has been proposed a two-unit zoom lens having a variable magnification ratio of 1.5 or thereabout in which the first lens unit is composed of positive, negative, positive and positive lenses, and the second lens unit is composed of two lenses, i.e., positive and negative lenses, or three lenses, i.e., positive, negative and negative lenses.
Furthermore, in Japanese Laid-Open Patent Applications No. Hei 9-211325 and No. Hei 10-48523, there has been proposed a small-sized zoom lens having a high optical performance in which the first lens unit is composed of a positive lens, a negative lens, an aspherical lens and a positive lens, the second lens unit is composed of a positive lens and a negative lens, and the aspherical lens is effectively disposed.
In the above-described two-unit zoom lens composed of two lens units, i.e., a first lens unit of positive refractive power and a second lens unit of negative refractive power, in order to obtain a high optical performance throughout the entire variable magnification range while minimizing the entire lens system, it is necessary to appropriately set a refractive power of each lens unit.
In general, if the refractive power of each of the first and second lens units is strengthened, it becomes easy to obtain a high variable magnification, and the amount of movement of each lens unit during the variation of magnification becomes small, so that it becomes possible to shorten the total length of the entire lens system. However, if the refractive power of each lens unit is merely strengthened, the variation of aberrations due to the variation of magnification becomes large, so that it becomes difficult to correct the variation of aberrations well.
For example, in the above-mentioned Japanese Laid-Open Patent Applications No. Sho 63-311224, No. Hei 4-161914, No. Sho 62-90611, No. Sho 62-113120 and No. Hei 3-116110, since the refractive power of each lens unit is relatively weak, the distance from the first lens surface to the last lens surface in the telephoto end tends to become long.
Further, in the above-mentioned Japanese Laid-Open Patent Application No. Hei 9-211325, a plastic aspherical lens is used in each lens unit, thereby attaining a zoom lens which is easy to produce, but the distance from the first lens surface to the last lens surface in the telephoto end tends to become long.
The present invention has been made to improve the conventional two-unit zoom lenses, and an object of the invention is to provide a zoom lens having a high optical performance and having a simple construction while attaining the minimization of the entire lens system, in particular, attaining the thin lens system.
To attain the above object, in accordance with a first aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, the first lens unit comprising, in order from the object side to the image side, a first lens of positive refractive power having a convex surface facing the object side, a second lens of negative refractive power, a third lens of positive refractive power of meniscus form having a convex surface facing the image side and having an aspheric surface, and a fourth lens of positive refractive power, and a second lens unit of negative refractive power, wherein a variation of magnification is effected by varying an interval between the first lens unit and the second lens unit, and the following conditions are satisfied:
0.40 less than f1/fw less than 0.80xe2x80x83xe2x80x83(1)
0.40 less than |f2|/fw less than 0.90xe2x80x83xe2x80x83(2)
where fw is a focal length of the zoom lens in a wide-angle end, f1 is a focal length of the first lens unit, and f2 is a focal length of the second lens unit.
Further, in accordance with a second aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, the first lens unit consisting of, in order from the object side to the image side, a first lens subunit, a stop, and a second lens subunit of positive refractive power having an aspherical lens, and a second lens unit of negative refractive power, wherein a variation of magnification is effected by varying an interval between the first lens unit and the second lens unit, and the following conditions are satisfied:
0.40 less than f1/fw less than 0.80xe2x80x83xe2x80x83(9)
0.40 less than |f2|/fw less than 0.70xe2x80x83xe2x80x83(10)
1.46 less than N13 less than 1.62xe2x80x83xe2x80x83(11)
|fw/f13| less than 0.60xe2x80x83xe2x80x83(12)
0.40 less than Lp/D1 less than 0.70xe2x80x83xe2x80x83(13)
where f1 and f2 are focal lengths of the first lens unit and the second lens unit, respectively, fw is a focal length of the zoom lens at a wide-angle end, N13 is a refractive index of material of the aspherical lens included in the second lens subunit, f13 is a focal length of the aspherical lens, D1 is a total length of the first lens unit, and Lp is a distance between the stop and a lens surface disposed on the most image side of the first lens unit.
Further, in accordance with a third aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, the first lens unit consisting of, in order from the object side to the image side, a first lens subunit consisting of a first lens of positive refractive power and a second lens of negative refractive power, a stop, and a second lens subunit consisting of a third lens made of plastic material and having an aspheric surface and a fourth lens of positive refractive power, and a second lens unit of negative refractive power, wherein a variation of magnification is effected by varying an interval between the first lens unit and the second lens unit, and the following conditions are satisfied:
0.40 less than f1/fw less than 0.80xe2x80x83xe2x80x83(9)
0.40 less than |f2|/fw less than 0.70xe2x80x83xe2x80x83(10)
1.46 less than N13 less than 1.62xe2x80x83xe2x80x83(11)
|fw/f13| less than 0.60xe2x80x83xe2x80x83(12)
0.40 less than Lp/D1 less than 0.70xe2x80x83xe2x80x83(13)
0.01 less than d34/d1 less than 0.10xe2x80x83xe2x80x83(14)
where f1 and f2 are focal lengths of the first lens unit and the second lens unit, respectively, fw is a focal length of the zoom lens at a wide-angle end, N13 is a refractive index of material of the third lens, f13 is a focal length of the third lens, D1 is a total length of the first lens unit, Lp is a distance between the stop and a lens surface disposed on the most image side of the first lens unit, and d34 is an air separation between the third lens and the fourth lens.
Further, in accordance with a fourth aspect of the invention, there is provided an optical apparatus comprising a zoom lens according to any one of the first to third aspects of the invention.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.